Geosynthetics June/July 2021 - 36

Dewatering of sludge from a sewage treatment plant with geotextile tubes
correctly and clearly indicated by marking
on the geotextile surface in an easily
viewable location. The indicated height
must be strictly adhered to, as it represents
the means of the tensile forces
acting on the tube, the maximum allowable
height for safety, and the correct
functionality of the dewatering system.
If the maximum height is not adhered
to and is exceeded for any reason, there
is a high risk of tube rupture. This risk
is increased by the gain of height with
its filling, due to the increase in internal
pressure, and it should be avoided.
As the geotextile tube is made by a
FIGURE 4 Dredger pumping sludge for the dewatering process
0.008 inches (0.2 mm), speed of normal
flow through the plane of the geotextile
(NBR 11.058 2013) of 20.10-3 m/s and
resistance to tensile strength (NBR 10.319
2013), both longitudinal and transversal,
of 600 pounds-force per inch (105 kN/m).
The sludge was then pumped through
FIGURE 5 Geotextile tubes in the
dewatering process
a dredger, removing the densified material
from the ponds and filling the geotextile
tubes located in the drying beds (Figure
4). The geotextile tubes received several
filling cycles until their maximum admitted
volume was reached. After this process,
they rested for a period of three weeks.
Through the dewatering process, the
sludge increased from 5% solid content by
mass to approximately 25% inside the geotextile
tubes. This represents a reduction
from the initial 423,776 cubic feet (12,000
m³) to 49,441 cubic feet (1,400 m³) at the
end. The process provides 8.5 times less
volume than the original, resulting in lower
transportation and final disposal costs.
Considerations about
the dewatering process
The maximum height that a geotextile
dewatering tube can reach must be
36
Geosynthetics | June July 2021
filtering element, over time the water will
percolate, and the tube will lose height
due to consolidation. In this way, new
filling cycles are possible, treating and
dewatering larger volumes of sludge.
Thus, a geotextile tube can receive a
sludge volume many times larger than
its nominal volume.
The filling of a geotextile tube with
predominantly organic sludge provides
biological clogging of the geotextile pores.
This clogging comes from the formation
of bacterial and algal colonies that blind
the geotextile, preventing water percolation
and, thus, retaining moisture inside
the tube. This clogging is easily avoided,
however; for example, in the case studied,
frequent washes of the pipe surface were
performed with a pressure hose.
Conclusion
Through the application case presented,
it is possible to realize that dewatering in
drying beds would not provide the effective
cleaning and dewatering of the sludge
from the consolidation ponds in time.
Furthermore, mechanical dewatering
techniques, which would be effective for
the purpose of cleaning the ponds over
time, were not viable, representing high
investment and maintenance costs for
the service.
Geosynthetics June/July 2021

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